Question: On the first 7 quizzes of his chemistry class, Luis got an average score of 84. What does he need on the next quiz to have an overall average of 86?
Solution: Let his score on the next quiz be $x$ The sum of all of his scores is then $7 \cdot 84 + x$ The same sum must also be equal to $8 \cdot 86$ Solve: $x = 8 \cdot 86 - 7 \cdot 84 = 100$.